Can't we all just get along?

Oh wise internet I seek your wisdom once again. So a female friend of mine recently reconnected with a guy friend of hers from her high school. So they're talking and catching up when he admits to her that he has always liked, her that since high school he has still liked her and that whole deal. As she is telling me about him she says that they have a lot in common and that, that scares her. Why does it scare her? Because he knows her so well, just by looking into her eyes even and what she wants is someone who'll be a challenge. Will challenge her, someone she'll butt heads with.

Now here is where I am confused. Is a person understanding you and having lots in common with you necessarily a bad thing? If not when does it become a bad thing? I don't know to me, if you guys can work well together in a relationship and butt heads as little as possible it seems like a good thing. I don't know that is just me. What do you think?